Method for Manufacturing an Optical Fiber Preform

ABSTRACT

A method for manufacturing an optical fiber preform includes the steps of depositing an inner cladding and a central core inside a fluorine doped silica tube and thereafter collapsing the silica tube to form a primary preform. The fluorine doped silica tube has a cross section area that is no more than about 15 percent smaller than the cross section area of the resulting primary preform. The present method facilitates reduced-cost manufacturing of a high-capacity optical fiber preform, which may be drawn to produce an optical fiber having reduced transmission losses.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application is a continuation of commonly assigned U.S. patentapplication Ser. No. 11/627,483 for a Method for Manufacturing anOptical Fiber Preform, filed Jan. 26, 2007, (and published Feb. 7, 2008,as Publication No. 2008/0031582 A1), which itself claims the benefit ofFrench Patent Application No. 06/00754 (filed Jan. 27, 2006, at theNational Institute of Industrial Property (France)). Each of theforegoing patent applications and patent application publication ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing an opticalfiber preform.

BACKGROUND OF THE INVENTION

An optical fiber is made by drawing a preform on a drawing tower. Apreform for example comprises a primary preform consisting of a veryhigh quality glass tube, part of the cladding and the core of the fiber.This primary preform is then overcladded or sleeved to increase itsdiameter and to form a preform which can be used on a drawing tower. Inthis context, the term inner cladding is used for the cladding formedinside the tube, and outer cladding for the cladding formed on theoutside of the tube. The homothetic fiber drawing operation consists ofplacing the preform vertically in a tower and drawing a fiber strandfrom one end of the preform. For this purpose a high temperature isapplied locally to one end of the preform until the silica is softened,the fiber drawing speed and temperature then being permanentlycontrolled during the draw operation since they determine the diameterof the fiber.

The geometry of the preform must strictly comply with the ratios of therefractive indexes and diameters of the core and the fiber cladding sothat the drawn fiber has the required profile. For optical fibers, theindex profile is generally qualified in relation to the graph tracing ofthe function which associates the refractive index with the radius ofthe fiber. Conventionally the distance r to the center of the fiber isshown along the abscissa axis, and the difference between the refractiveindex and the refractive index of the outer cladding of the fiber isshown along the ordinate axis. The index profile is therefore referredto as a “step,” “trapezoidal” or “triangular” index as per therespective graph tracings of step, trapezoidal or triangular shape.These curves generally represent the theoretical or set profile of thefiber, the manufacturing constraints of the fiber possibly leading to aslightly different profile.

An optical fiber conventionally consists of an optical core whosefunction is to transmit and optionally amplify an optical signal, and ofan optical cladding whose function is to confine the optical signalwithin the core. For this purpose the refractive indexes of the coren_(c) and of the cladding n_(g) are such that n_(c)>n_(g). As is wellknown, the propagation of an optical signal in a single-mode opticalfiber decomposes into a fundamental mode guided within the core and intosecondary modes guided over a certain distance in the core-claddingassembly, called cladding modes.

As line fiber for optic fiber transmission systems, single mode fibers(SMF) are conventionally used. An SMF fiber conventionally has a core ofgermanium-doped silica to increase its refractive index, and a claddingof pure silica. To improve attenuation in an optical fiber it is knownto reduce the quantity of dopant in the core. However, since thedifference in index between the core and the cladding is fixed by thedesired propagation properties of the optical fiber, the index of thecladding must then be reduced or at least the index of the innercladding which is doped with fluorine for example. The conditionn_(c)>n_(g) for the refractive indexes of the core n_(c) and claddingn_(g) must be met to ensure guiding of the optical signal along thefiber.

Fibers with pure silica cores are also known, and are called Pure SilicaCore Fibers (PSCF). The absence of dopant in the core of a PSCF fibermakes it possible to limit optical losses. A PSCF fiber thereforeconventionally has a cladding in silica doped with fluorine to reduceits refractive index. A PSCF fiber can be manufactured from a preformcomprising a primary preform consisting of a tube, generally in quartz,in which one or more layers of fluorine-doped silica have been depositedto form an inner cladding, and one or more layers of pure silica havebeen deposited to form the central core of the fiber. After depositingthe layers corresponding to the core and the inner cladding, the tube isclosed onto itself during an operation called collapsing. In this waythe primary preform is obtained. This primary preform is thenovercladded, generally with natural silica particles for cost-relatedreasons.

A conventional PSCF fiber, or an optical fiber having a central corescarcely doped with germanium with a cladding doped with fluorine,therefore has an index profile with a central core of radius a and indexn_(c) corresponding to the index of silica or slightly higher than thatof silica, and an inner buried cladding of outer radius b. The innercladding is said to be buried since it has a refractive index n_(g) thatis less than that of the outer cladding n_(e) obtained by theovercladding or sleeving of the primary preform. This outer cladding isgenerally of pure silica glass and has substantially the same refractiveindex as the central core in a PSCF fiber.

In the above-described structure, with an outer cladding havingsubstantially the same refractive index as the central core, thefundamental mode is not completely guided and shows additional losses,called leakage. To minimize these leakage losses, the percentage ofenergy propagating in the outer, pure silica cladding must be reduced.The ratio between the outer radius of the fluorine-doped inner claddingand the radius of the core (b/a) must therefore be sufficiently high;i.e. the inner cladding of doped silica must be extended at least as faras a critical radius b whose value is dependent on the core radius andon the difference Δn=n_(c)−n_(g) between the core index and the index ofthe inner cladding; for a single mode fiber it is considered that aratio between the radius of the inner cladding and the radius of thecore that is 8 or more (b/a>8) ensures good confinement of the opticalsignal in the central core and an acceptable level of leakage losses.

To enlarge the outer diameter of the fluorine-doped cladding, documentJP 55100233 proposed the use of a tube of fluorine-doped silica tomanufacture the primary preform.

Also the capacity of a preform is defined as the quantity of opticalfiber length which can be drawn from this preform. The greater thediameter of the preform, the greater this capacity. To reducemanufacturing costs, it is desirable to provide long lengths of linearfibers from one same preform. It is therefore sought to fabricatepreforms of large diameter while complying with the above-mentionedconstraints regarding the diameters of the central core and thefluorine-doped cladding.

In this context, either the ratio between the outer radius of theprimary preform and the radius of the central core is relatively highand the quantity of silica to be deposited inside the tube is high, inwhich case the primary preform is costly and the method is not veryproductive; or the ratio between the outer radius of the tube and theradius of the central core is relatively low and the optical fiberobtained by drawing from the final preform does not have good propertiesand its attenuation is substantially higher.

EP 1 544 175 proposes making a preform with a part of the outer claddingin fluorine-doped silica in order to increase the total diameter of thefluorine-doped cladding without increasing the diameter of the costlyprimary preform. The primary preform is obtained by successive depositsof layers of doped silica in a tube of fluorine-doped silica, and thenthis primary preform is overcladded with a first layer of syntheticsilica particles doped with fluorine then with a layer of natural silicaparticles. Overcladding with synthetic silica particles doped withfluorine makes it possible to increase the diameter of thefluorine-doped cladding for one same core diameter, and reduce leakagelosses. The overclad of the primary preform using silica particles dopedwith fluorine is less costly than depositing fluorine-doped silicainside the tube. Nonetheless the fluorine-doped silica particles aresynthetic particles which are much more expensive than natural silicaparticles.

US Patent Application Publication No. 2002/0144521 is directed to amethod for manufacturing a preform of large capacity. This documentproposes making a primary preform by depositing a large diameter centralcore inside a tube doped with chlorine and fluorine. The tube is dopedwith fluorine to compensate for the increase in refractive indexgenerated by doping with chlorine. The tube is doped with chlorine tolimit the presence of OH groups which degrade the optical transmissionproperties in the central core. The use of said tube doped with chlorineand fluorine makes it possible to reduce the thickness of the innercladding deposited in the tube in order to produce a primary preformhaving an enlarged central core diameter. This primary preform is thenovercladded to obtain a final preform of large diameter and hence oflarge capacity. The tube doped with chlorine and fluorine protects thecentral core against impurities brought by the overcladding process withnatural silica particles. However, the refractive index of this tube issubstantially the same as that of pure silica.

The compromise between manufacturing a low cost preform having a largedrawing capacity for an optical fiber having reduced optical losses witha central core that is not or only scarcely doped and having an innercladding doped with fluorine still remains to be improved upon.

SUMMARY OF THE INVENTION

The invention therefore proposes using a tube of fluorine-doped silicawhich is sufficiently thick to limit the quantity of silica depositedinside the tube and to enable overcladding with natural silicaparticles, while guaranteeing a ratio between the diameter of thefluorine-doped cladding and the diameter of the core (b/a) that issufficiently high to ensure the confinement of the optical signal withinthe central core.

In particular, the invention proposes a method of manufacturing aprimary preform having a cross section area, the method including thesteps of making a deposit of an inner cladding and a central core insidea fluorine doped silica tube, followed by collapsing the thus obtainedtube to form the primary preform, wherein the tube being chosen suchthat its cross section area is maximally about 15%, for example,maximally about 10%, smaller than the cross section area of the primarypreform obtained from the tube after collapsing.

In addition, the present invention proposes a method of manufacturing anoptical fiber preform by overcladding the primary preform thus obtainedusing silica particles to obtain an overclad on the outer surface of theprimary preform.

In one embodiment, the invention proposes a method for manufacturing afinal optical fiber preform by overcladding a primary preform having apredetermined total cross section area, the method including at leastone manufacturing step of the primary preform by deposit of an innercladding and a central core inside a tube of fluorine-doped silica, thetube being chosen such that its cross section area is maximally about15%, for example maximally about 10%, smaller than the cross sectionarea of the primary preform.

According to one embodiment, the overcladding is carried out usingnatural silica particles.

According to one characteristic, the chosen tube has a cross sectionarea of more than about 700 mm².

According to one embodiment, the chosen tube has a cross section area ofless than about 1500 mm².

According to one embodiment, the chosen tube has a cross section area ofless than about 1000 mm².

According to one embodiment, the deposit inside the tube is controlledso that the ratio of the outer radius of the primary preform to theradius of the central core is superior or equal to about 8.

According to one embodiment, the deposit of the inner cladding is madeof silica doped with fluorine or silica doped with fluorine andgermanium.

According to one embodiment, the deposit of the central core is made ofsubstantially pure silica, silica slightly doped with germanium orsilica slightly doped with germanium and fluorine.

The invention also proposes a final optical fiber preform including aprimary preform with a predetermined cross section area and including atube in fluorine-doped silica having a cross section area that ismaximally about 15%, for example maximally about 10%, smaller than thetotal cross section area of the primary preform in which tube a depositof an inner cladding and a central core is present; and an overclad onthe outer surface of the primary preform.

According to one embodiment, the tube of silica contains fluorine at aconcentration of between about 1 and about 2% by weight.

According to one embodiment, the deposit of the inner cladding is ofsilica doped with fluorine or silica doped with fluorine and germanium.

According to one embodiment, the deposit of the central core is ofsubstantially pure silica, silica slightly doped with germanium orsilica slightly doped with germanium and fluorine.

According to one embodiment, the overclad is made of silica, forexample, natural silica.

According to one embodiment, the ratio of the outer diameter of theprimary preform to the diameter of the central core is superior or equalto about 8.

The present invention also relates to an optical fiber preform whereinthe preform includes from the center towards the periphery of a centralcore having a refractive index n_(c) and a radius a, a first part of theinner cladding formed by a deposit of an inner cladding having arefractive index n_(g1) and having an outer radius corresponding to Φi,a second part of the inner cladding formed by the tube having arefractive index n_(g2) and having an outer radius corresponding to Φe,and an outer cladding having a refractive index n_(e) having a radiusR_(oc), in which a<Φi<Φe<R_(oc).

According to one embodiment, the absolute value of the difference inrefractive index between the second part of the inner cladding formed bythe tube and the first part of the inner cladding formed by deposit ofthe inner cladding, being |n_(g2)−n_(g1)|, is equal to or less thanabout 10% of the difference in refractive index between the central coreand the second part of the inner cladding formed by the tube, being|n_(c)−n_(g2)|.

According to another embodiment, |n_(g2)−n_(g1)| is equal to or lessthan about 1% of |n_(c)−n_(g2)|.

The invention also concerns an optical fiber obtained by drawing of thefinal preform of the invention.

According to one embodiment, for a wavelength of about 1550 nm, theoptical fiber has losses of about 0.18 dB/km or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic cross-sectional view of a preform of theinvention;

FIG. 2 illustrates an example of a set index profile for an opticalfiber obtained by drawing of the preform of the invention related to aschematic cross-section view of a preform of the invention (not toscale); and

FIG. 3 illustrates another example of a set index profile for an opticalfiber obtained by drawing of the preform of the invention related to aschematic cross-section view of a preform of the invention (not toscale).

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter inthe following detailed description of the invention, in which some, butnot all embodiments of the invention are described. Indeed, thisinvention may be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will satisfy applicablelegal requirements.

The invention proposes a method for fabricating an optical fiberpreform.

A primary preform is made from a thick tube in silica doped withfluorine. As used herein the term “thick tube” refers to a tube having alarge wall thickness. The cross-section area of the tube is maximally(no more than) about 15%, for example maximally about 10%, less than thetotal cross section area of the primary preform, i.e. a large part ofthe inner cladding (the second part of the inner cladding) of theprimary preform consists of the thick tube doped with fluorine. A smallpart of the inner cladding of the primary preform (the first part of theinner cladding) is formed by the deposit of an inner cladding inside thetube. For a desired thickness of the inner cladding, the following isobserved. The larger the thickness of the wall of the tube forming theinner cladding, the smaller the required thickness of the deposit of theinner cladding. The quantity of deposit inside the tube is thereforelimited and a primary preform of large capacity can be fabricated at lowcost. The tube may have a cross section area more than about 700 mm²,for example about 900 mm², and a fluorine concentration of between about1 and about 2% by weight. Most of the tubes used up until now tomanufacture a primary preform have a cross section area of less than 400mm².

The primary preform is manufactured by successive deposits inside thetube of layers of fluorine-doped silica to form an inner cladding, andof layers of pure silica or slightly doped with germanium to form acentral core. The deposits inside the tube are of Chemical VaporDeposition (CVD) type. This type of deposit is made by injecting agaseous mixture inside the tube consisting of precursors such as silicontetrachloride (SiCl₄), germanium tetrachloride (GeCl₄), phosphorusoxychloride (POCl₃), silicon tetrafluoride (SiF₄) or hexafluoroethane(C₂F₆) and oxygen. Oxidation of the precursors makes it possible tosynthesize the different layers forming the core and inner cladding ofthe primary preform. CVD-type depositing encompasses Modified ChemicalVapor Deposition (MCVD), Furnace Chemical Vapor Deposition (FCVD), andPlasma Enhanced Chemical Vapor Deposition (PCVD).

After the deposit of the inner cladding and the central core, theprimary preform is collapsed. After collapsing, the obtained primarypreform is overcladded with silica particles, for example low costnatural silica particles to obtain a final preform. The overcladding ofthe final preform can be made by plasma deposit in which the (natural)silica particles are blown and fused by a plasma torch under atemperature in the region of about 2300° so that they are vitrified onthe periphery of the primary preform. The overcladding operation isgenerally conducted in a closed cabin under a controlled atmosphere toensure protection against electromagnetic disturbances and release ofozone emitted by the plasma torch.

FIG. 1 shows a cross-sectional view of the final optical fiber preformof the invention. The final optical fiber preform was obtained by anoverclad 16 of a primary preform 15. The primary preform 15 has a totalcross section area that is predetermined in relation to the desiredfiber drawing capacity. The primary preform 15 was fabricated by CVDdeposit 11 of layers of doped and/or substantially pure silica inside atube 10. According to the invention, the tube 10 used to manufacture theprimary preform 15 is a tube 10 of silica doped with fluorine, forexample at a concentration of between about 1 and about 2% by weight.Also, the tube 10 of fluorine-doped silica used in the method of theinvention is thick, i.e. has a large wall thickness, with a crosssection area designated by the abbreviation CSA that is maximally about15%, for example maximally about 10%, less than the total cross sectionarea of the primary preform 15. For example, the tube 10 may have across section area of more than about 700 mm² and less than about 1500mm², for example less than about 1000 mm². The cross section area isexpressed as follows:

CSA=π/4((Φe ² −Φi ²)

where Φe and Φi are the respective outer (exterior) and inner (interior)radii of the tube 10. R_(oc) denotes the radius of the outer cladding.

To obtain a CSA of high value, greater than about 700 mm² and less thanabout 1500 mm², for example less than 1000 mm², the tube 10 must haveboth a large outer diameter and a small inner diameter, i.e. it musthave a large wall thickness.

To fabricate an optical fiber with a central core that is not or onlyslightly doped with germanium, the presence of fluorine in the tube 10and the large wall thickness of the tube 10 make it possible to limitthe thickness of the inner cladding doped with fluorine to be depositedinside the tube 10, without it being necessary to deposit an outercladding doped with fluorine to guarantee the characteristics of signalpropagation within the core. The advantage of doping with fluorine isthat this reduces the influence of the water peak, especially in a PCVDprocess.

The small inner diameter Φi of the tube 10 makes it possible to limitthe thickness of the layers deposited inside the tube 10, CVD-typedeposits being relatively costly. Only the central core and a smallportion of the fluorine-doped inner cladding are deposited inside thetube 10. The large outer diameter Φe of the tube 10 allows the use ofnatural silica particles for the overclad 16 of the primary preform 15,which is less costly, while guaranteeing a large buried cladding on theoptical fiber obtained after fiber drawing.

It is therefore possible to fabricate primary preforms 15 with a largecentral core occupying a large portion of the layers 11 deposited insidethe tube 10, and hence to increase the fiber drawing capacity of thefinal optical fiber preform, without deteriorating the optical signalpropagating characteristics.

For example from a tube 10 having a cross section area of about 900 mm²,it is possible to manufacture a primary preform 15 having a crosssection area of about 1035 mm² by only depositing about 135 mm² linearof silica via CVD deposit 11. The cost of manufacturing the primarypreform 15 is therefore reduced. More generally, the cross section areaof the primary preform 15 will only be about 10% to about 15% greaterthan the cross section area of the tube 10. The quantity of deposit 11inside the tube 10 is therefore limited in the method of the invention.In addition, from the primary preform 15, having a cross section area inthe order of about 1035 mm², it is possible, after overcladding 16 isperformed, to obtain a final optical fiber preform of large diameterenabling the drawing of a large linear quantity of fiber, i.e.approximately 250 km of fiber for a final optical fiber preform of about1 meter.

The thick tube 10 of fluorine-doped silica may be fabricated using theso-called sol-gel technique known as such, which consists of fabricatinga silica gel which is molded into tube form and dried. A rigid butporous tube is obtained which is densified in a stream of hot air. Afluorine dopant may be added directly to the silica gel or when heatingthe tube dried by injection of a fluorinated gas.

FIG. 2 shows a set index profile of the fiber obtained by homotheticdrawing of the present optical fiber preform. This set index profile isrelated in FIG. 2 to a cross-sectional view of a preform according tothe present invention (not to scale).

In FIG. 2 the central core can be seen of radius a and refractive indexn_(c), substantially equal to the refractive index of the outer claddingn_(e) which corresponds to the index of pure silica. The core may be ofsubstantially pure or slightly doped silica and the outer cladding canbe of substantially pure silica for the cost-related reasons mentionedabove. An inner cladding of refractive index n_(g) separates the corefrom the outer cladding. The inner cladding is buried, i.e. itsrefractive index n_(g) is lower than the index of the outer claddingn_(e). This condition is laid down by the fact that the central core hasa refractive index n_(c) close to that of pure silica and therelationship n_(c)>n_(g) must be maintained to guarantee the propagationof the optical signal.

For that purpose, the inner cladding is of fluorine-doped silica. Asindicated above, the inner cladding of the fiber corresponds to the zoneof the present preform covering the tube 10 and the inner claddingdeposited inside the tube 10. More specifically, the buried claddingcomprises layers of fluorine-doped silica deposited by PCVD deposit forexample in the tube 10 and the tube 10 itself.

It is shown in the drawing of FIG. 2 the core radius a, the radius ofthe inner cladding deposited in the tube 10 and which corresponds to theinner radius of the tube 10 Φi and the outer radius of the tube 10 Φewhich corresponds to the outer radius b of the fluorine-doped cladding.The outer radius b of the fluorine-doped cladding therefore correspondsto the outer radius of the primary preform 15 obtained by themanufacturing method of the invention before overcladding. Thereforeaccording to the definitions given previously, the ratio b/a or Φe/a isabout 8 or more. The optical signal propagating in a fiber having theindex profile of FIG. 2 will therefore be effectively confined in thecentral core whether slightly doped or non-doped.

In FIG. 3 another embodiment of the present invention is shown. FIG. 3shows another set index profile of the fiber obtained by homotheticdrawing of the present optical fiber preform. This set index profile isrelated in FIG. 3 to a cross-sectional view of a preform according tothe present invention (not to scale). In this embodiment the refractiveindex of the inner cladding consists of two levels, firstly therefractive index n_(g1) of the small or first part of the inner claddingformed by the deposit 11 of the inner cladding and secondly therefractive index n_(g2) of the large or second part of the innercladding formed by tube 10. The two levels can be the same, beingn_(g1)=n_(g2). However, it is possible that small variations occurduring depositing of the inner cladding inside the tube 10. The absolutevalue of the difference in refractive index between the inner claddingformed by the tube 10 and the deposit 11 of the inner cladding, viz.between n_(g2) and n_(g1) (|n_(g2)−n_(g1)|) can be equal to or less thanabout 10%, for example about 1%, of the difference in refractive indexbetween the inner cladding formed by the tube 10 and the deposit 11 ofthe inner cladding, viz. between n_(c) and n_(g2) (|n_(c)−n_(g2)|). Thismeans that the level of n_(g1) may be slightly above or slightly belowthe level of n_(g2).

The optical losses in a fiber drawn from the inventive preform will beabout 0.18 dB/km or less for a transmission wavelength of about 1550 nm.With the method of the invention, it is therefore possible tomanufacture a preform of large capacity at reduced cost which allows thedrawing of an optical fiber having reduced transmission losses.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed and that modifications andother embodiments are intended to be included within the scope of theappended claims. Although specific terms are employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being defined in the claims.

1. A method of manufacturing a primary preform having a cross sectionarea, the method comprising the steps of: depositing an inner claddingand a central core inside a fluorine doped silica tube; and thereaftercollapsing the thus obtained tube to form the primary preform; whereinthe fluorine doped silica tube has a cross section area no more thanabout 15 percent smaller than the cross section area of the primarypreform obtained from the tube after collapsing.
 2. The method accordingto claim 1, wherein the cross section area of the fluorine doped silicatube is no more than about 10 percent smaller than the cross sectionarea of the primary preform.
 3. The method according to claim 1, whereinthe step of depositing an inner cladding and a central core inside afluorine doped silica tube is controlled in such a manner that the ratioof the outer radius of the primary preform to the radius of the centralcore is greater than or equal to about
 8. 4. The method according toclaim 1, wherein the inner cladding comprises silica doped with fluorineor silica doped with fluorine and germanium.
 5. The method according toclaim 1, wherein the central core comprises substantially pure silica,silica doped with germanium, or silica doped with germanium andfluorine.
 6. The method according to claim 1, wherein the fluorine dopedsilica tube has a cross section area of more than about 700 mm².
 7. Themethod according to claim 6, wherein the fluorine doped silica tube hasa cross section area of less than about 1500 mm².
 8. The methodaccording to claim 1, wherein the fluorine doped silica tube has a crosssection area of less than about 1000 mm².
 9. The method according toclaim 1, wherein the inner cladding and central core have a combinedcross section area of no more than about 135 mm².
 10. A method ofmanufacturing an optical fiber preform comprising the steps of:depositing an inner cladding and a central core inside a fluorine dopedsilica tube, wherein the central core consists essentially ofsubstantially pure silica; and thereafter collapsing the thus obtainedtube to form a primary preform; wherein the fluorine doped silica tubehas a cross section area no more than about 15 percent smaller than thecross section area of the primary preform obtained from the tube aftercollapsing.
 11. The method according to claim 10, wherein the crosssection area of the fluorine doped silica tube is no more than about 10percent smaller than the cross section area of the primary preform. 12.The method according to claim 10, wherein the step of depositing aninner cladding and a central core inside a fluorine doped silica tube iscontrolled in such a manner that the ratio of the outer radius of theprimary preform to the radius of the central core is greater than orequal to about
 8. 13. The method according to claim 10, wherein theinner cladding comprises silica doped with fluorine or silica doped withfluorine and germanium.
 14. The method according to claim 10, whereinthe fluorine doped silica tube has a cross section area of more thanabout 700 mm².
 15. The method according to claim 10, wherein thefluorine doped silica tube has a cross section area of less than about1500 mm².
 16. The method according to claim 10, wherein the fluorinedoped silica tube has a cross section area of less than about 1000 mm².17. The method according to claim 10, wherein the deposited innercladding and central core have a combined cross section area of no morethan about 135 mm².
 18. The method according to claim 10, wherein thestep of depositing an inner cladding and a central core inside afluorine doped silica tube is performed using Plasma Enhanced ChemicalVapor Deposition.